1. Field of Invention
The present invention relates to optical display apparatus, in particular switchable active lenses for display apparatus. Such a switchable active lens may be used to provide: a switchable two dimensional (2D)/three dimensional (3D) autostereoscopic display apparatus; a switchable high brightness reflective display system; or a multi-user display system. Such display systems may be used in computer monitors, telecommunications handsets, digital cameras, laptop and desktop computers, games apparatuses, automotive and other mobile display applications.
2. Description of Related Art
3D Displays
Normal human vision is stereoscopic, that is each eye sees a slightly different image of the world. The brain fuses the two images (referred to as the stereo pair) to give the sensation of depth. Three dimensional stereoscopic displays replay a separate, generally planar, image to each of the eyes corresponding to that which would be seen if viewing a real world scene. The brain again fuses the stereo pair to give the appearance of depth in the image.
FIG. 1a shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 3 on the display plane and a left eye 4 views a left eye homologous point 5 on the display plane to produce an apparent image point 6 perceived by the user behind the screen plane.
FIG. 1b shows in plan view a display surface in a display plane 1. A right eye 2 views a right eye homologous image point 7 on the display plane and a left eye 4 views a left eye homologous point 8 on the display plane to produce an apparent image point 9 in front of the screen plane.
FIG. 1c shows the appearance of the left eye image 10 and right eye image 11. The homologous point 5 in the left eye image 10 is positioned on a reference line 12. The corresponding homologous point 3 in the right eye image 11 is at a different relative position 3 with respect to the reference line 12. The separation 13 of the point 3 from the reference line 12 is called the disparity and in this case is a positive disparity for points which will lie behind the screen plane.
For a generalised point in the scene there is a corresponding point in each image of the stereo pair as shown in FIG. 1a. These points are termed the homologous points. The relative separation of the homologous points between the two images is termed the disparity; points with zero disparity correspond to points at the depth plane of the display. FIG. 1b shows that points with uncrossed disparity appear behind the display and FIG. 1c shows that points with crossed disparity appear in front of the display. The magnitude of the separation of the homologous points, the distance to the observer, and the observer's interocular separation gives the amount of depth perceived on the display.
Stereoscopic type displays are well known in the prior art and refer to displays in which some kind of viewing aid is worn by the user to substantially separate the views sent to the left and right eyes. For example, the viewing aid may be colour filters in which the images are colour coded (e.g. red and green); polarising glasses in which the images are encoded in orthogonal polarisation states; or shutter glasses in which the views are encoded as a temporal sequence of images in synchronisation with the opening of the shutters of the glasses.
Autostereoscopic displays operate without viewing aids worn by the observer. In autostereoscopic displays, each of the views can be seen from a limited region in space as illustrated in FIG. 2.
FIG. 2a shows a display device 16 with an attached parallax optical element 17. The display device produces a right eye image 18 for the right eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 19 to produce a right eye viewing window 20 in the region in front of the display. An observer places their right eye 22 at the position of the window 20. The position of the left eye viewing window 24 is shown for reference. The viewing window 20 may also be referred to as a vertically extended optical pupil.
FIG. 2b shows the left eye optical system. The display device 16 produces a left eye image 26 for the left eye channel. The parallax optical element 17 directs light in a direction shown by the arrow 28 to produce a left eye viewing window 30 in the region in front of the display. An observer places their left eye 32 at the position of the window 30. The position of the right eye viewing window 20 is shown for reference.
The system comprises a display and an optical steering mechanism. The light from the left image 26 is sent to a limited region in front of the display, referred to as the viewing window 30. If an eye 32 is placed at the position of the viewing window 30 then the observer sees the appropriate image 26 across the whole of the display 16. Similarly the optical system sends the light intended for the right image 18 to a separate window 20. If the observer places their right eye 22 in that window then the right eye image will be seen across the whole of the display. Generally, the light from either image may be considered to have been optically steered (i.e. directed) into a respective directional distribution.
FIG. 3 shows in plan view a display device 16, 17 in a display plane 34 producing the left eye viewing windows 36, 37, 38 and right eye viewing windows 39, 40, 41 in the window plane 42. The separation of the window plane from the display is termed the nominal viewing distance 43. The windows 37, 40 in the central position with respect to the display are in the zeroth lobe 44. Windows 36, 39 to the right of the zeroth lobe 44 are in the +1 lobe 46, while windows 38, 41 to the left of the zeroth lobe are in the −1 lobe 48.
The viewing window plane of the display represents the distance from the display at which the lateral viewing freedom is greatest. For points away from the window plane, there is a diamond shaped autostereoscopic viewing zone, as illustrated in plan view in FIG. 3. As can be seen, the light from each of the points across the display is beamed in a cone of finite width to the viewing windows. The width of the cone may be defined as the angular width.
If an eye is placed in each of a pair viewing zones such as 37, 40 then an autostereoscopic image will be seen across the whole area of the display. To a first order, the longitudinal viewing freedom of the display is determined by the length of these viewing zones.
The variation in intensity 50 across the window plane of a display (constituting one tangible form of a directional distribution of the light) is shown with respect to position 51 for idealised windows in FIG. 4a. The right eye window position intensity distribution 52 corresponds to the window 41 in FIG. 3, and intensity distribution 53 corresponds to the window 37, intensity distribution 54 corresponds to the window 40 and intensity distribution 55 corresponds to the window 36.
FIG. 4b shows the intensity distribution with position schematically for more realistic windows. The right eye window position intensity distribution 56 corresponds to the window 41 in FIG. 3, and intensity distribution 57 corresponds to the window 37, intensity distribution 58 corresponds to the window 40 and intensity distribution 59 corresponds to the window 36.
The quality of the separation of images and the extent of the lateral and longitudinal viewing freedom of the display is determined by the window quality, as illustrated in FIG. 4. FIG. 4a shows the ideal viewing windows while FIG. 4b is a schematic of the actual viewing windows that may be outputted from the display. Several artefacts can occur due to inadequate window performance. Cross talk occurs when light from the right eye image is seen by the left eye and vice versa. This is a significant 3D image degradation mechanism which can lead to visual strain for the user. Additionally, poor window quality will lead to a reduction in the viewing freedom of the observer. The optical system is designed to optimised the performance of the viewing windows.
The parallax optical element may be a parallax barrier. The display comprises a backlight, an array of electronically adjustable pixels (known as a Spatial Light Modulator, SLM) arranged in columns and rows and a parallax barrier attached to the front of the display as illustrated in plan view in FIG. 5.
Parallax barriers rely on blocking the light from regions of the display and therefore reduce the brightness and device efficiency, generally to approximately 20-40% of the original display brightness. Parallax barriers are not readily removed and replaced due to the requirements of sub-pixel alignment tolerances of the barrier with respect to the pixel structure of the display in order to optimise the viewing freedom of the display. The 2D mode is half resolution.
Another type of parallax optic (cf. parallax barriers) well known in the art for use in stereoscopic displays is called the lenticular screen, which is an array of vertically extended cylindrical microlenses. The term “cylindrical” as used herein has its normal meaning in the art and includes not only strictly spherical lens shapes but also aspherical lens shapes. The pitch of the lenses corresponds to the viewpoint correction condition, that is the pitch of the parallax barrier is slightly smaller than twice the pitch of the pixel array in order to steer the light from each pixel to the viewing window. In such a display, the resolution of each of the stereo pair images is half the horizontal resolution of the base LCD, and two views are created.
The curvature of the lenses is set substantially so as to produce an image of the LCD pixels at the window plane. As the lenses collect the light in a cone from the pixel and distribute it to the windows, lenticular displays have the full brightness of the base panel.
FIG. 6 shows a typical structure for a lenticular display device using a lenticular array. A backlight 60 produces a light output 62 which is incident on an LCD input polariser 64. The light is transmitted through a TFT LCD substrate 66 and is incident on a repeating array of pixels arranged in columns and rows in an LCD pixel plane 67. The red pixels 68, 71, 73, green pixels 69, 72, 75 and blue pixels 70, 73 each comprise an individually controllable liquid crystal layer and are separated by regions of an opaque mask called a black mask 76. Each pixel comprises a transmissive region, or pixel aperture 78. Light passing through the pixel is modulated in phase by the liquid crystal material in the LCD pixel plane 74 and in colour by a colour filter positioned on an LCD colour filter substrate 80. The light then passes through an output polariser 82 after which is placed a parallax barrier 84 and a parallax barrier substrate 86. The parallax barrier 84 comprises an array of vertically extended transmissive regions separated by vertically extended opaque regions and serves to direct light from alternate pixel columns 69, 71, 73, 75 to the right eye as shown by the ray 88 for light from pixel 69 and from the intermediate columns 68, 70, 72, 74 to the left eye as shown by the ray 90 (this overall light direction pattern forming another example of a directional distribution of light). The observer sees the light from the underlying pixel illuminating the aperture of the barrier, 92. The light then passes through a lenticular screen substrate 94 and a lenticular screen 96 which is formed on the surface of the lenticular screen substrate 92. As for the parallax barrier, the lenticular screen 94 serves to direct light from alternate pixel columns 69, 71, 73, 75 to the right eye as shown by the ray 88 from the pixel 69 and from the intermediate columns 68, 70, 72, 74 to the left eye as shown by the ray 90 from pixel 68. The observer sees the light from the underlying pixel illuminating the aperture of the individual lenticule, 98 of the lenticular screen 96. The extent of the captured light cone is shown by the captured rays 100.
Lenticular displays are described in T. Okoshi “Three Dimensional Imaging Techniques”, Academic Press, 1976. One type of lenticular display using a spatial light modulator is described in U.S. Pat. No. 4,959,641, in particular non-switching lenticular elements in air.
A lenticular display using cylindrical lenses that are tilted with respect to columns of pixels of a display is described in “multiview 3D—LCD” published in SPIE Proceedings Vol. 2653, 1996, pages 32 to 39.
The viewing freedom of the flat panel displays described above is limited by the window structure of the display.
A display in which the viewing freedom is enhanced by measuring the position of an observer and moving the parallax element in correspondence is described in EP-0,829,743. Such an observer measurement apparatus and mechanical actuation is expensive and complex.
A display in which the window optical structure is not varied (a fixed parallax optic display for example) and the image data is switched in correspondence to the measured position of the observer such that the observer maintains a substantially orthoscopic image is described for example in EP-0,721,131.
As described above, the use of parallax optics to generate a spatially multiplexed 3D display limits the resolution of each image to at best half of the full display resolution. In many applications, the display is intended to be used for a fraction of the time in the 3D mode, and is required to have a full resolution artefact free 2D mode.
One type of display in which the effect of the parallax optic is removed is Proc. SPIE vol. 1915 Stereoscopic Displays and Applications IV (1993) pp 177-186, “Developments in Autostereoscopic Technology at Dimension Technologies Inc.”, 1993. In this case, a switchable diffuser element is placed in the optical system used to form the light lines. Such a switchable diffuser could be for example of the Polymer Dispersed Liquid Crystal type in which the molecular arrangement switches between a scattering and non-scattering mode on the application of an applied voltage across the material. In the 3D mode, the diffuser is clear and light lines are produced to create the rear parallax barrier effect. In the 2D mode, the diffuser is scattering and the light lines are washed out, creating the effect of a uniform light source. In this way, the output of the display is substantially Lambertian and the windows are washed out. An observer will then see the display as a full resolution 2D display. Such a display suffers from Fresnel diffraction artefacts in the 3D mode, as well as from unwanted residual scatter in the diffuser's clear state which will increase the display cross-talk. Therefore, such a display is likely to exhibit higher levels of visual strain.
In another type of switchable 2D-3D display disclosed in EP-0,833,183, a second LCD is placed in front of the display to serve as a parallax optic. In a first mode, the parallax LCD is clear so that no windows are produced and an image is seen in 2D. In a second mode, the device is switched so as to produce slits of a parallax barrier. Output windows are then created and the image appears to be 3D. Such a display has increased cost and complexity due to the use of two LCD elements as well as being of reduced brightness or having increased power consumption. If used in a reflective mode 3D display system, parallax barriers result in very poor brightness due to attenuation of light by the blocking regions of the parallax barrier both on the way in and out of the display.
In another type of switchable 2D-3D display disclosed in EP-0,829,744, a parallax barrier comprises a patterned array of halfwave retarder elements. The pattern of retarder elements corresponds to the pattern of barrier slits and absorbing regions in a parallax barrier element. In a 3D mode of operation, a polariser is added to the display so as to analyse the slits of the patterned retarder. In this way, an absorbing parallax barrier is produced. In the 2D mode of operation, the polariser is completely removed as there is no involvement of any polarisation characteristics in the 2D mode of operation. Thus the output of the display is full resolution and full brightness. One disadvantage is that such a display uses parallax barrier technology and thus is limited to perhaps 20-30% brightness in the 3D mode of operation. Also, the display will have a viewing freedom and cross talk which is limited by the diffraction from the apertures of the barrier.
It is known to provide electrically switchable birefringent lenses for purposes of switching light directionally. It is known to use such lenses to switch a display between a 2D mode of operation and a 3D mode of operation.
For example, electrically switchable birefringent liquid crystal microlenses are described in European Optical Society Topical Meetings Digest Series: 13, 15-16 May 1997 L. G. Commander et al “Electrode designs for tuneable microlenses” pp 48-58.
In another type of switchable 2D-3D display disclosed in U.S. Pat. No. 6,069,650 and WO-98/21620, switchable microlenses comprising a lenticular screen filled with liquid crystal material are used to change the optical power of a lenticular screen. U.S. Pat. No. 6,069,650 and WO-98/21620 teach the use of an electro-optic material in a lenticular screen whose refractive index is switchable by selective application of an electric potential between a first value whereby the light output directing action of the lenticular means is provided and a second value whereby the light output directing action is removed.
A 3D display comprising a liquid crystal Fresnel lens is described in S. Suyama et al “3D Display System with Dual Frequency Liquid Crystal Varifocal Lens”, SID 97 DIGEST pp 273-276.